Precancerous conditions of the anus

Precancerous conditions of the anus are changes to anal cells that make them more like cancer. These conditions are not yet cancer. But if they are not treated, there is a chance that they will become anal cancer.

When the cells of the anus start to change, it is called anal intraepithelial neoplasia, or AIN. These areas may also be called squamous cell intraepithelial lesions (SILs) because most of the cell changes start in the squamous cells in the lining of the anus.

AIN is given a grade between 1 and 3, which describes how abnormal the cells look under the microscope.

AIN 1 means that the cells are slightly abnormal compared to normal cells.
AIN 2 means that the cells are somewhat or moderately abnormal when compared to normal cells.
AIN 3 means that the cells are very abnormal when compared to normal cells.

Precancerous changes to the anus may also be described as high grade or low grade.

Low-grade squamous cell intraepithelial lesions (LSILs) are made up of cells that look very much like normal cells. They often return to normal on their own, so there is a low risk that they will develop into anal cancer. This is equal to AIN 1.

High-grade squamous cell intraepithelial lesions (HSILs) are made up of cells that look very abnormal under the microscope. They usually don't return to normal. These changes are at high risk of progressing to anal cancer if they are not treated. This is equal to AIN 2 or AIN 3. These high-grade lesions are also called carcinoma in situ or Bowen disease.

Risks

The most important risk for developing a precancerous condition of the anus is infection with the human papillomavirus (HPV). The following risks increase the chance that an HPV infection will not go away on its own and may develop into a precancerous condition:

human immunodeficiency virus (HIV) infection
a weakened immune system caused by drugs given after an organ transplant
a history of vulvar intraepithelial neoplasia (VIN), which is a precancerous condition of the vulva
a history of cervical intraepithelial neoplasia (CIN), which is a precancerous condition of the cervix

Find out about HPV and cancer.

Symptoms

Precancerous conditions of the anus do not usually cause any symptoms. Most people do not know they have AIN or SILs. If there are any symptoms, they may include:

itching
discharge
raised or scaly areas around the anus
warts

Diagnosis

If you have symptoms or your doctor thinks you might have a precancerous condition of the anus, you will be sent for tests. Diagnostic tests may include:

an anal swab to collect a sample of cells to be examined under a microscope (called anal cytology)
an anoscopy ( endoscopy of the anus)
a biopsy

Treatments

Treatment options for AIN or SILs include:

surgery, electrosurgery or laser surgery
topical creams such as fluorouracil (Efudex), imiquimod (Aldara) or trichloroacetic acid (TCA)
photodynamic therapy (PDT)
watchful waiting

Ahmed S, Eng C, Messick CA. Squamous cell carcinoma of the anal canal. Yalcin S, Phillip PA (eds.). Textbook of Gastrointestinal Oncology. Springer Nature Switzerland; 2019: 10:171–180.
Hoedema RE. Anal intraepithelial neoplasia and squamous cell cancer of the anus. Clinics in Colon and Rectal Surgery. 2018: 31(6):347–352.
Siddharthan RV, Lanciault C, Tskitis VL. Anal intraepithelial neoplasia: diagnosis, screening and treatment. Annals of Gastroenterology. 2019: 32(3):257–263.

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